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Knitting Science (1)

Knitting Science (1). Jimmy Lam Institute of Textiles & Clothing. Learning Objectives. Aspect of Knitting Science Relaxation and shrinkage Fabric geometry and K-value Cover Factor HARTA Research Ref: Machine Knitting and Fabrics by K.F. Au, Chapter 24, Plain Knit Fabric Geometry.

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Knitting Science (1)

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  1. Knitting Science (1) Jimmy Lam Institute of Textiles & Clothing

  2. Learning Objectives • Aspect of Knitting Science • Relaxation and shrinkage • Fabric geometry and K-value • Cover Factor • HARTA Research Ref: Machine Knitting and Fabrics by K.F. Au, Chapter 24, Plain Knit Fabric Geometry

  3. Aspect of Knitting Science • Using a scientific approach to study knitted fabric especially with knitted garment size variation. • Three Basic Laws were established on knitted fabric behavior (by Hatra Research) • Loop length is the fundamental unit of weft knitting structure; • Loop shape determines the dimensions of the fabric and this shape depends upon yarn used, treatment and fabric received; • The relationship between loop shape and loop length may be expressed in the form of simple equations

  4. Relaxation and shrinkage • Knitted fabric tends to change in width and length when taken from machine, even without yarn shrinkage, indicates a change in loop shape rather than loop length • During knitting, the loop structure is subjected to tension from take down mechanism. This “stressed” fabric will relax once more favorable conditions provided • e.g during washing and wearing with results in change of dimension and customer satisfaction.

  5. Knitted fabric geometry • Two fundamental factors for fabric geometry theory: • the stitch length, i.e the length in the knitted loop, is the dominant factor in all structure. • There are seversal dimensionally stable states possible for a knitted structure.

  6. Relaxation States • The three “relaxed states” for knitted fabrics are:- • Dry relaxed states: the fabric has been taken off the knitting machine and in course of time, attains a dimensionally stable condition. • Wet relaxed states: if the fabric is socked in water and allowed to dry flat, the wet-relaxed state is attached, again a dimensionally stable condition; • Finished relaxed states: to reach this stable condition, the fabric is subjected to agitation in water or steam, and a denser fabric results.

  7. Doyle’s experiment • The stitch length and fabric dimension when under dry, relaxed state is constant and is independent of yarn type, yarn count, machine type and machine gauge.

  8. K-factor (1) • Doyle showed that (from expt. Data) for a range of dry, relaxed plain weft fabric, the stitch density S is S=Ks/l2 • where S is stitch density, and, • l is loop length, and, • Ks is constant and independent of yarn and machine variable.

  9. K-factor (2) • Munden showed that (1959) there are linear dimension as well as stitch density for a wide range of relaxed plain fabrics. He suggested, the dimensions of plain fabric : • cpi = Kc/l; • wpi = Kw/l; • S=Ks/l2; and • cpi/wpi=Kc/Kw=R where R is loop shape factor

  10. K-factor (3) • Knapton further works on “K” value for different “relaxed state” fabric as follows:

  11. Cover Factor • Cover factor is ratio of the area covered by the yarn in one loop to the area occupied by that loop. • Area of yarn in one loop = l x d • Area taken by one loop = 1/cpi X 1/wpi = 1/S • Since S=Ks/l2; • Cover factor = l x d x S = l x d x Ks/l2 • assume d = Sqrt(tex) • Cover factor = Sqrt (tex)/l

  12. HATRA Research

  13. HATRA Research • Hoisery and Allied Trade Research Association (HATRA) shows that the quality, weight and dimensions of fabric as it comes off the knitting machines are affected by FOUR factors. They are : • dimensional and gauge of machine; • yarn count and quality; • take-down tension; and • length of yarn knitted into the stitch

  14. Fabric Quality • First, what is fabric quality ? The general trade interpretation is that two fabrics are of same quality when • their weights per unit area are the same; • their course per inch and wales spacing are similar; • their handles are identical; and • their elastic properties are the same.

  15. Fabric Quality (2) • For two fabrics to conform to these four requirements, and be of identical quality; when • the fabrics are knitted from yarns of similar count and quality; • the constructional details of the fabrics are identical • So the only factors affecting two fabric’s identical quality are : • take down tensions on the machine; • length of yarn knitted into a stitch

  16. Effect of take down tension • It have been showed that the higher the fabric take down tension, the narrower the fabric width and the lower value of course per inch. • However, this effect is only temporary on fabric dimensions and quality; these difference will disappear when fabric is thoroughly relaxed such as after wetting out or washing

  17. Conclusion • HATRA Research proves that • The last thing affects the fabric quality is stitch length.

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